How do you get the CRM Account from a Project record

How do you get the CRM Account from a Project record

Through the GUI web interface its possible to see that Projects are linked to the customer Account in CRM that the project is for, however I can't see that link / account ID in the data returned from the API (eg from GET  /portal/[PORTALID]/projects/[PROJECTID]/).

Is this done through Entity Properties?  In which case what should the entity_id parameter be set to?

Thanks
Simon